1. Field of the Invention
The present invention is directed to optically pure (-) clethodim, novel compositions of matter containing (-) clethodim substantially free from (+) clethodim, and methods of controlling vegetative growth employing such compositions.
2. Description of the Related Art
Undesired vegetative growth in an agricultural setting can greatly impact the ultimate yield of crop plants. More commonly referred to as weeds, such growth can deplete the available water and nutrients available to desired plants, thereby inhibiting the growth of the desired plant and reducing the yield of useful plant materials. The use of herbicides to control plant growth has proven to be a successful means of countering the deleterious effects that weeds may have on crop growth.
Certain cyclohexanedione oximes are known in the art as having excellent herbicidal activity against a variety of post-emergent grasses in a variety of environments. Examples of cyclohexanedione oximes include clethodim, sethoxydim, cycloxydim, alloxydim, tralkoxydim, tepraloxydim, and clefoxydim. These compounds are characterized by a single ring structure that involves keto functions in the 1 and 3 positions with an oxime function in the 2 position. The significance of the structural similarity is revealed in the mode of action of these compounds, which involves chelation of metal ions associated with plant enzymes, otherwise responsible for promoting necessary biochemical reactions in the plant. This binding effect involves the oxime side chain and the enolic form of the 1,3-diketone to form a six-member ring incorporating the metal ion.
The 5 position substitutions of three of the cyclohexanedione oximes, clethodim, sethoxydim, and cycloxydim, contains a chiral carbon atom. As with many organic compounds, these three cyclohexanedione oximes exist in optically active forms, i.e., they have the ability to rotate the plane of plane-polarized light. In describing an optically active compound, the prefixes d and 1 or s and r are used to denote the absolute configuration of the molecule about its chiral center(s). The prefixes d and 1, or (+) and (-) respectively, are employed to designate the sign of rotation of plane-polarized light by the compound, with (-) or 1 meaning that the compound is levorotatory. A compound prefixed with (+) or d is dextrorotatory. For a given chemical structure, these compounds, called stereoisomers, are identical except that they are mirror images of one another. A specific stereoisomer may also be referred to as an enantiomer, and a mixture of such enantiomers is often called an enantiomeric or racemic mixture. Enhanced activity of a single enantiomer, over that of a racemic mixture and the other opposite enantiomer, has not been taught or suggested based on studied modes of action of these compounds.
The active compound of the present invention is an optical isomer of the compound clethodim. Clethodim and related compounds are described in U.S. Pat. No. 4,440,566. The generic chemical structure of clethodim is shown below in formula I: ##STR1##
Clethodim (generic name), or Select.RTM. (trade name) (Valent U.S.A. Corp.), is a particularly important commercial herbicide within the class of cyclohexanedione oximes characterized by a 5 position substitution. It is generally systematically applied to crop plants, such as soybeans, so that the growth of grass weeds growing in the plot can be controlled. Select.RTM. is commercially produced as a composition comprising 26.4% clethodim by weight and 73.6% other ingredients, referred to as Select.RTM. 2EC (Valent U.S.A. Corp.), reflecting that it contains 2 pounds active ingredient (clethodim) per gallon. However, clethodim has only been used in the past as the 1:1 racemic mixture. That is, it is available only as a mixture of optical isomers. To applicants' knowledge, the individual isomers have not been isolated in pure or substantially pure form prior to the present invention.
While many effective herbicides, such as clethodim, have been developed, those skilled in the art will recognize that there is a need for herbicides with greater selectivity and improved effectiveness over current compounds. Use of such improved herbicides will result in decreased damage to non-target plants and reduced application rates, thereby reducing environmental effects and costs. This invention is directed to these, and other important ends.